Cathode saturation arrangement for fuel cell power plant

ABSTRACT

The heat from various portions of a fuel cell power plant ( 110 ) are redistributed in a manner allowing desired modification of/to the heat removal means ( 152,156 ), e.g., radiator ( 152 ), included in the coolant loop for the fuel cell stack assembly (CSA) ( 12 ). A humidifier ( 70 ) added in the coolant loop ( 114 ) and the inlet oxidant (air) stream ( 134 ′) serves to relatively increase the humidification of the inlet air while removing heat from the coolant prior to entering the CSA ( 12 ). The combined effects are to relatively increase the temperature of the coolant exiting the CSA without similarly increasing the temperature of the coolant entering the CSA, and to relatively increase the temperature differential (“pinch”) between the coolant entering the heat removal means and the cooling air of the heat removal means ( 152, 156 ). This latter effect permits a relative reduction in the size/capacity of the heat removal means ( 152, 156 ).

This application is a continuation of U.S. patent application Ser. No.11,787,570 filed Apr. 17, 2007, which in turn is a continuation of U.S.patent application Ser. No. 11/327,912 filed Jan. 9, 2006, which is inturn a divisional of U.S. patent application Ser. No. 10/723,081 filedNov. 26, 2003, now U.S. Pat. No. 7,014,933.

TECHNICAL FIELD

This invention relates to fuel cell power plants, and particularly tothe management of heat in a fuel cell power plant. More particularlystill, the invention relates to a fuel cell cathode saturationarrangement for managing heat loads in a fuel cell power plant designedfor volume optimization.

BACKGROUND ART

Fuel cell power plants are commonly used to produce electrical energyfrom reducing and oxidizing fluids to power electrical apparatus. Insuch power plants, one or typically a plurality, of planar fuel cellsare arranged in a fuel cell stack, or cell stack assembly (CSA). Eachcell generally includes an anode electrode and a cathode electrodeseparated by an electrolyte. A reducing fluid such as hydrogen issupplied to the anode electrode, and an oxidant such as oxygen or air issupplied to the cathode electrode. The reducing fluid and the oxidantare typically delivered to and removed from the cell stack viarespective manifolds. In a cell using a proton exchange membrane (PEM)as the electrolyte, the hydrogen electrochemically reacts at a catalystsurface of the anode electrode to produce hydrogen ions and electrons.The electrons are conducted to an external load circuit and thenreturned to the cathode electrode, while the hydrogen ions transferthrough the electrolyte to the cathode electrode, where they react withthe oxidant and electrons to produce water and release thermal energy.

The anode and cathode electrodes of such fuel cells are separated bydifferent types of electrolytes, depending on operating requirements andlimitations of the working environment of the fuel cell. One suchelectrolyte is a PEM electrolyte, which consists of a solid polymer wellknown in the art. Other common electrolytes used in fuel cells includephosphoric acid, sulfuric acid, or potassium hydroxide held within aporous, non-conductive matrix between the anode and cathode electrodes.It has been found that PEM cells have substantial advantages over cellswith liquid acid or alkaline electrolytes in satisfying specificoperating parameters because the membrane of the PEM provides a barrierbetween the reducing fluid and oxidant that is more tolerant to pressuredifferentials, is fixed and cannot be leached from the cell, and has arelatively stable capacity for water retention.

In operation of PEM fuel cells, it is usually desirable that a properwater balance be maintained between the rate at which water is producedat the cathode electrode including water resulting from proton dragthrough the PEM electrolyte and the rate at which water is removed fromthe cathode and anode electrodes. This is to prevent excessive drying orflooding of one or more of the various elements of the fuel cell.

In addition to water balance in the fuel cell power plant, there is thefurther requirement of a coolant system for maintaining appropriatetemperature of the various components of the power plant. Typically,though not necessarily, the coolant will also be the water discussedabove with respect to the need for water balance. The coolant istypically used to remove heat from certain portions of the fuel cellpower plant, as for instance the fuel cell stack assembly (CSA), thoughthe coolant may in some instances serve also as a source of heat. Thecoolant may also serve as a source of moisture for the control ofhumidification of various gas streams in the fuel cell power plant. Inthese ways the coolant serves to address the various heat loads ofvarious portions of the fuel cell power plant.

The CSA may include a coolant plate means, or the like, that defines acoolant channel through the cell stack assembly, typically adjacent tothe cathode, and which forms part of a coolant loop that is bothinternal and external to the CSA. The coolant loop typically includes atleast a circulation means, such as a pump, and some form of heat removalmeans, such as a radiator. Inasmuch as the electrochemical reaction inthe CSA may be the source of considerable heat, the coolant serves theimportant role of removing heat from the CSA. Coolant entering the CSAadjacent to the exiting cathode exhaust serves to cool the exhauststream and condense water out of that gas stream, through the use offine pore media such as the coolant plate means that define the coolantchannel adjacent the cathode exhaust. The amount of heat removed is afunction of the coolant temperature and flow rate of the coolantentering the CSA.

Because the coolant is recirculated in the coolant loop, the heatremoval means performs the important function of removing, prior to itsreintroduction to the CSA, most of the heat acquired during thecoolant's passage through the CSA. While the heat removal means mighttake a variety of forms, by far the most common is that of an air-cooledradiator. Typically, it is the task of the radiator to remove all of theheat acquired by the coolant's passage through the CSA. The air whichcools the radiator is typically at some ambient temperature associatedwith the environment of the fuel cell power plant, and may typically be,or approach, 120° F. (49° C.), particularly if the CSA is being used ina hot environment such as a desert. Because the temperature of thecoolant exiting the CSA is not substantially greater than that of theradiator-cooling air, or stated conversely, because the temperature ofthe radiator-cooling air may be only a little less than that of thecoolant exiting the CSA, the resulting relatively small temperaturedifferential, sometimes referred to as the “pinch”, requires that thecapacity of the radiator be relatively large in order to achieve thenecessary cooling. On the other hand, this relative largeness of theradiator may be objectionable for several reasons, including initialcost, weight, size, appearance, and costs associated with its operationand maintenance.

Thus it is desirable to provide a fuel cell power plant in which theheat is managed in a manner allowing for a relative reduction in thesizing of the heat removal means, such as a radiator.

DISCLOSURE OF INVENTION

The heat and/or heat loads of various devices or portions of a fuel cellpower plant are redistributed or re-allocated in a manner allowingdesired modification of/to the heat removal means included in thecoolant loop for the fuel cell stack assembly (CSA). The addition of ahumidifier in the coolant loop and the inlet oxidant (air) stream servesto relatively increase the humidification of the inlet air whileremoving heat from the coolant prior to entering the CSA. The combinedeffects are to relatively increase the temperature of the coolantexiting the CSA without similarly increasing the temperature of thecoolant entering the CSA, and further to relatively increase thetemperature differential (“pinch”) between the coolant entering the heatremoval means and the cooling air of the heat removal means. This lattereffect permits a relative reduction in the size/capacity of the heatremoval means required.

In a fuel cell power plant, there is provided a fuel cell stack assembly(CSA), a coolant loop including a heat removal means, operativelyassociated with the CSA, and a humidifier operatively connected in thecoolant loop. The CSA includes an anode region having an inlet and anoutlet, a cathode region having an inlet and an outlet, an electrolyteregion intermediate the anode and cathode regions, and a coolant regionhaving an inlet and an outlet connected in the coolant loop. An inletfuel stream is connected to the anode region inlet. An inlet oxidantstream is operatively connected to the cathode region inlet via thehumidifier. The heat removal means may typically be a radiator, activelycooled by a medium such as air having a temperature somewhat less thanthat of the coolant from the CSA. The inlet oxidant stream is passedthrough the humidifier before entering the cathode of the CSA, and inthe humidifier becomes at least partially, and typically heavily,humidified by mass and heat transfer association with the coolant alsobeing passed through the humidifier. The humidifier needs to allow massand heat transfer between two fluid streams, as via an energy recoverydevice (ERD). The ERD may preferably be of the type in which a fine poremedium separates the two streams but allows fluid transfer therebetween,or alternatively may be a bubble or contact saturator or the like inwhich there is direct contact between the two fluid streams without thepresence of an intermediate porous barrier.

The foregoing features and advantages of the present invention willbecome more apparent in light of the following detailed description ofexemplary embodiments thereof as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a simplified schematic diagram of a fuel cell power plant inaccordance with the prior art, illustrating examples of temperatures atselected portions of the plant including the fuel cell stack assembly(CSA) and the coolant loop;

FIG. 2 is a simplified graphic view of the evaporation/condensationprofile in a standard fuel cell for an air stream that is not highlyhumidified;

FIG. 3 is a simplified graphic view of the evaporation/condensationprofile in a standard fuel cell for an air stream that is highlyhumidified; and

FIG. 4 is a simplified schematic diagram similar to FIG. 1, of a fuelcell power plant in accordance with the invention, illustrating theinclusion of a humidifier for humidifying the inlet oxidant and furthercooling the coolant, and illustrating examples of temperatures atselected portions of the plant.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, there is depicted in simplified schematic diagramform a fuel cell power plant 10 in accordance with the prior art,indicating representative temperatures at selected portions of theplant, including a fuel cell stack assembly (CSA) 12 and a coolant loop14. The fuel cell power plant 10 includes a number of fuel cellsarranged in a known manner in a fuel cell stack assembly 12. Althoughthe illustration of CSA 12 in FIG. 1 is intended to embrace plural fuelcells, it is depicted as a single cell for ease of illustration andreference. Each fuel cell, and thus the CSA 12, typically includes ananode region 16, a cathode region 18, an electrolyte region 20 betweenthe anode region 16 and the cathode region 18, and a coolant region orcoolant channel 22, typically adjacent the cathode region 18. Theelectrolyte is preferably a proton exchange membrane (PEM) typeemploying a solid polymer well known in the art.

In the fuel cell power plant 10, a reducing agent in the form of ahydrogen-rich fuel stream 24 is supplied to the anode region 16, as atanode inlet 26. The various anode, cathode, and coolant region inletsand outlets mentioned hereinafter are typically in the form of manifoldsserving the respective regions. The hydrogen-rich fuel stream 24 istypically derived from a hydrocarbon fuel source 28 by means of a fuelprocessing system (FPS) 30 of known design. A source of oxidant 32, suchas air, supplies an oxidant stream 34 to the cathode region 18 viacathode inlet 36, which flow may be assisted in a known manner by amotive device, such as a fan, compressor, or blower 38.

After passing through the anode region 16, partially-depleted fuel isdischarged or exhausted as anode exhaust stream 40 at anode outlet 42.After passing through the cathode region 18, partially-depleted oxidantis exhausted as cathode exhaust stream 44 at cathode outlet 46.

The coolant channel 22 is an included portion of the coolant loop 14,and may be defined by a sealed or a porous coolant plate, not separatelyshown. When characterized as a sealed plate, the coolant, typicallywater, may not flow between the coolant channel 22 and the cathodeand/or anode regions 18, 16. On the other hand, when characterized as afine-pore, porous plate, the coolant water and product water may passbetween the coolant channel 22 and the cathode and/or anode regions 18,16 via the pores. In either event, water in the coolant loop 14 entersthe coolant channel 22 of CSA 12 at inlet 48 and returns to the loop viaoutlet 50. The coolant loop 14 is here shown in simplified form ascontaining a heat removal means, such as radiator 52, and acirculation-assistance means, such as pump 54. A coolant loop portion14′ connects coolant channel outlet 50 with pump 54, a portion 14″connects pump 54 with the inlet to radiator 52, and a further portion14′″ connects the outlet of radiator 52 to the coolant channel inlet 48.The radiator 52 includes a motorized fan 56 for forcing air through, oracross, the radiator to effect heat transfer between that air and theliquid coolant passing through the radiator and the coolant loop 14. Thecoolant loop 14 may additionally include a variety of means, not shown,for cleansing, degasifying, adding and/or discharging, and/or otherwiseprocessing the coolant, as is known in the art.

In the conventional power plant configuration depicted in FIG. 1, theradiator 52 may need to be relatively large to provide the amount ofcooling needed to remove heat acquired during the coolant's passagethrough the CSA 12. The function of the coolant as it passes through theCSA 12 is to remove heat generated by the fuel cell reaction.

The sizing of radiator 52 is primarily governed by the difference intemperature between the coolant exiting the CSA 12 on coolant loopportions 14′, and 14″ and thus entering the radiator, and the air beingdelivered by fan 56 to remove heat from the coolant in the radiator. Inthis consideration, the coolant outlet 50 of CSA 12 may be viewed as the“Source” of heat and the air-cooled radiator 52 as the “Sink”. As thedifference between these two temperatures (source temperature and sinktemperature) gets smaller, or closer, the radiator size and fan powerrequirements get larger, and in a non-linear manner, such that a smalldecrease in that temperature differential may result in a relativelymuch larger change (increase) in the size of the radiator 52.Conversely, a small increase in that temperature differential may enablea significant decrease in the size of the radiator 52, other thingsbeing relatively constant. This result is accomplished by adjusting theoperating points, and thus the energy concentration, or temperature, atvarious points in the system. Reference will be made to initialoperating conditions and temperatures in FIG. 1 as being representativeof the prior art. It should be understood that the temperaturesmentioned herein with respect to FIGS. 1-4 are meant to be arbitrary andmerely exemplary and in no way limiting, and are cited principally forillustrative comparative purposes. If it is assumed the ambient air at,or in, the fuel cell power plant 10 is about 120° F. (49° C.), as in aworst case operating condition, then it will be seen that both the air(oxidant) stream 34 entering the cathode 18 and the air at fan 56 beingused to cool radiator are at 120° F. (49° C.). For this explanation ofFIG. 1, assume that the coolant entering coolant channel 22 at coolantinlet 48 has a temperature of about 135° F. (57° C.), and thetemperature of the coolant exiting the coolant channel 22 at outlet 50will have a temperature of about 160° F. (71° C.). Thus, it is necessaryto obtain a 25° F. (14° C.) coolant temperature drop across the radiator52 with Source temperature-to-Sink temperature difference of only 40° F.(22° C.), thereby requiring a relatively large radiator and fan.

At this juncture it is useful to consider the evaporation/condensationprofiles of a typical fuel cell in which, first, the cathode air streamis not highly humidified, as seen in FIG. 2, and secondly, the cathodeair stream is highly, or at least relatively more highly, humidified, orsaturated, as seen in FIG. 3. It should be understood when referring toFIGS. 2 and 3 that some graphic license is used in depicting theinterior of a fuel cell, especially the cathode air stream and thecoolant flow path, and although the orientations are dissimilar fromFIG. 1, the appropriate reference numerals have been used.

Referring first to FIG. 2, the coolant enters the CSA 12 at coolantinlet 48, which is along the edge, or in the area, where the cathode airexits, as at cathode outlet 46. The coolant serves to cool the cathodeexhaust stream there and condense water out of the cathode exhaust. Thatcondensed water is represented quantitatively and locationally by thedense hatching 58. Correspondingly, the heat is removed from the cathodegas stream and enters the coolant. The amount of heat removed is afunction of the coolant inlet 48 temperature and flow rate to the CSA.For the same flow rate, a reduction in coolant temperature results inmore water being condensed. In a representative fuel cell power plant,sufficient water is condensed to maintain water balance in the powerplant 10, including the FPS 30 and the coolant loop 14, if the coolantflow is sufficient, for example 45 pph/cell, and the coolant temperatureis low enough, for example 135° F. (57° C.). Thus, regardless ofactivity in the remainder of the coolant loop 14 of FIG. 1, if thecoolant inlet 48 meets the aforementioned conditions, then the powerplant shall be in water balance.

Referring further to FIG. 2, the coolant exits the CSA 12 at coolantoutlet 50, which is along the edge, or in the area, where the air entersthe cathode 18, as at cathode inlet 36. The heated coolant water servesto heat and, to some extent, humidify this cathode air stream becausethe CSA 12 in this respect operates in a manner analogous to an energyrecovery device. That humidification occurs in an evaporation region ofthe fuel cell that is represented quantitatively and locationally by thesimple hatching 60. In this evaporation region 60, the heat is removedfrom the coolant and enters the cathode gas stream. The temperature ofthe coolant correspondingly drops such that, in the aforementionedrepresentative example, the coolant temperature at the coolant outlet isabout 160° F. (71° C.). These temperatures are consistent with thosedepicted in the system of FIG. 1.

Reference is now made to FIG. 3, which is similar in most respects toFIG. 2, but in which the air stream entering the cathode 18 at cathodeinlet 36 has already been humidified to a dew point approximate to theinlet temperature of the CSA 12, such that relatively less heat andwater is required to complete the humidification to local operatingconditions of the CSA 12. This difference is depicted by the evaporationregion 60 in FIG. 3 being relatively smaller than it was in FIG. 2, andresults in a higher-grade (temperature) heat exiting the fuel cell (orCSA 16) in the coolant at coolant outlet 50. For the example mentionedabove, if the dew point of the air stream entering the cathode inlet 36were now 130° F. (54.5° C.) as a result of its increased humidificationprior to that point, the coolant temperature at coolant exit 50 wouldincrease to about 165° F. (74° C.).

Simply raising the temperature of the coolant exiting the fuel cell/CSA12 at coolant outlet 50 does not, in and of itself, accomplish theobjective of being able to reduce the radiator size. This is principallybecause it would also relatively raise the temperature of the coolantentering coolant inlet 48, which runs counter to the discussion abovewhich required that temperature to remain at about 135° F. (57° C.).However, the process of partially humidifying the air stream fordelivery to the cathode inlet 36 overcomes that obstacle. Based on theexample discussed-above, by humidifying the air stream to a dew point of135° F. (57° C.), there results the transfer of heat equivalent to theremoval of more than 5° F. (3″ C) from the coolant. This is attained bythe addition of a humidifying device in accordance with the invention.

Referring to FIG. 4, there is depicted a fuel cell power plant 110 inaccordance with the invention. Reference numbers identical to those ofFIG. 1 are used in FIG. 4 for those components that are the same, orsubstantially the same, in the two configurations. However, where thereis some functional, compositional, or structural difference occasionedby the invention, but the components of FIG. 4 nevertheless remainanalogous to components in FIG. 1, they have been given the samereference number, but preceded by a “1”. The following description willemphasize the novel character, structure, and/or function of thecontaminant removal system of the invention, and will attempt tominimize repetition of description that is duplicative of that providedwith respect to FIG. 1.

While the fuel cell power plant 110 of FIG. 4 is similar in mostrespects to the power plant 10 described with respect to FIG. 1, itdiffers in at least the important aspect that the addition of ahumidifying device 70 enables the use of relatively smaller, simplerheat removal devices, in the form of a relatively smaller radiator 152and motorized fan 156. The humidifying device 70 is a relatively simple,small, and inexpensive device, and may typically take the form of anenergy recovery device (ERD) having a gas flow chamber 72 and a liquid,or coolant, flow chamber 74 separated by an enthalpy exchange barrier 76therebetween. The humidifying device 70, hereinafter also referred to as“ERD 70”, may be of any generally known construction in which an oxidant(air) stream and a coolant (water) stream may be passed in relative heatand mass transfer relation for relatively increasing the dewpoint/humidity of the air entering the cathode 18 of CSA 12 while alsoremoving heat from the coolant to be entering the coolant channel 22 ofthe CSA. It is preferred that the ERD 70 be sufficiently compact, simpleand inexpensive to offset those aspects of the prior radiator 52 and/ormotorized fan 56 relative to the radiator 152 and/or motorized fan 156replacing them. A preferred ERD 70 is of the type having adjacent gasand liquid chambers, 72 and 74 respectively, separated by a fine poresaturator medium, typically of graphite of the like, forming theenthalpy exchange barrier 76. A detailed description of one sucharrangement may be found in U.S. Pat. No. 6,274,259 to Grasso, et al andassigned to the assignee of the present invention, though the presentinvention may not require the inclusion of that patent's transfer mediumloop for wetting the enthalpy exchange barrier 76 herein. Otheracceptable barriers may be of the type similar to the fine pore, watertransfer plates such as used in/for the coolant channels within the CSA12. An alternative form of humidifier or energy recovery device, 70, maybe a bubble or contact saturator (not separately shown) in which theoxidant (air) stream is brought directly into contact with the coolant(water), as in a tank, reservoir, conduit, or the like, to effect therequisite transfer of mass and energy between the two fluids withoutrequiring that transfer to occur indirectly via an intermediate enthalpyexchange barrier.

The humidifying device 70 is inserted into the coolant loop 114relatively downstream of the radiator 152 and relatively upstream of thecoolant inlet 48 to the coolant channel 22 in CSA 12. Similarly, thehumidifying device 70 is in the inlet oxidant stream between the oxidantsource 32 and the cathode inlet 36 to the cathode 18 of CSA 12. Acoolant loop portion 114′″ connects the outlet, or discharge end, ofradiator 152 to the inlet end of coolant flow chamber 74 of ERD 70, anda coolant loop portion 114″″ connects the outlet end of that coolantflow chamber to the coolant inlet 48 of CSA 12. An oxidant conduitportion 134 connects oxidant from blower 38 to the inlet end of the gasflow chamber 72 of ERD 70, and a further oxidant conduit portion 134′connects the outlet end of that gas flow chamber to the cathode inlet 36of CSA 12. It will be understood that the blower 38 may be locatedeither prior to the inlet or after the outlet, of gas flow chamber 72.It is generally desirable for the air in gas flow chamber 72 and thewater in coolant flow chamber 74 to flow in counter flow relation to oneanother for maximum efficiency of the ERD, though other configurationsare within the scope of the invention.

Referring further to the operation of power plant 110 with the inclusionof the humidifying device 70, it is now possible to both relativelyincrease the dew point/humidity of the oxidant prior to its entry intocathode 18 and further cool the coolant leaving the radiator 152 priorto its entry into coolant channel 22. This results in the redistributionof heat in the power plant, and particularly the CSA 12, the coolantloop 114, and the fan 156 and radiator 152 within the coolant loop. Thisredistribution of the heat is illustrated by a comparison of thetemperatures at various locations in the power plant 110 of FIG. 4relative to the temperatures at similar locations in the power plant ofFIG. 1. It is now seen that the humidification of the oxidant prior toits introduction to cathode 18 results in a temperature of about 130° F.(54.5° C.) on conduit 134′ at the cathode inlet 36, which in turnrequires less heat from the coolant to complete the humidificationprocess in CSA 12 and thus results in a higher grade heat, i.e., 165° F.(74° C.), as the source temperature of the coolant exiting CSA 12 atcoolant outlet 50. This higher-grade heat in the coolant similarlyappears in coolant loop portion 14″ at the inlet to the radiator 152and, because the ambient air from fan 156 remains at 120° F. (49° C.)(the sink temperature), the temperature difference therebetween isrelatively increased. This enables the radiator 152 to effect the sameamount of cooling, i.e., a drop of about 25° F. (14° C.) to 140° F. (60°C.) at its outlet on loop portion 114′″, with a radiator of relativelysmaller capacity than that required for the same temperature drop acrossthe radiator 52 of FIG. 1. This is accomplished because even though thetemperature of the coolant leaving radiator 152 is 5° F. (3° C.) higherthan the 135° F. (57° C.) temperature desired for the coolant enteringthe coolant channel 22 at inlet 48, that desired temperature (135° F.,57° C.) is attained when the coolant from coolant loop portion 114′″passes through the coolant flow chamber 74 of humidifier 70 and exitsinto coolant loop portion 114″″. That further cooling of the coolantoccurs as the result of the heat removed therefrom during the oxidanthumidification process in ERD 70, as described earlier.

In view of the forgoing discussion, it will be appreciated that the fuelcell power plant 110 may be operated as satisfactorily as was the powerplant 10, yet with a relatively smaller and simpler radiator152/motorized fan 156 than was the case in power plant 10, and at anadditional “cost” of only a relatively simple, compact, and inexpensiveERD/humidifier 70. On balance, the reduced size and cost of the radiator152/fan 156 is typically a net advantage over any increased cost andsize of the added humidifier 70.

Although the invention has been described and illustrated with respectto the exemplary embodiments thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions and additions may be made without departing from the spiritand scope of the invention. For example, the radiator 52 of the heatremoval means may be any of numerous types of heat exchangers, e.g.,liquid to liquid, liquid to gas, etc. Moreover, the humidifying device70 may take various forms, including the fine pore saturator medium ofan ERD, the water transfer plates as in a CSA, a bubble saturator, orthe like, as well as others.

1. In a fuel cell power plant (110) including a fuel cell stack assembly(12); an inlet oxidant stream (134,134′) operatively connected to a fuelcell stack assembly oxidant region inlet (36); a coolant loop (114)operatively connected to a fuel cell stack assembly coolant region inlet(48) and outlet (50), the coolant loop (114) including a radiator (152,156) configured as a sink to transfer heat from a fuel cell stackassembly coolant at a source temperature to a sink temperature lowerthan the source temperature, the difference between said sourcetemperature and said sink temperature being a temperature differential,a method of relatively increasing said temperature differentialcomprising the steps of: cooling (74) the coolant in the coolant loop(114) by heat and mass transfer subsequent to passing the radiator (152,156) and prior to return introduction of the coolant to the fuel cellstack assembly (12); and relatively increasing the temperature andhumidity (72) of the inlet oxidant stream (134′) prior to introductionof the inlet oxidant stream to the fuel cell stack assembly oxidantregion inlet (36), thereby to distribute the heat of at least the fuelcell stack assembly (12) and the radiator (152, 156) so as to relativelyincrease the coolant exit temperature from the fuel cell stack assembly(12) and to the radiator (152, 156) so as to relatively increase saidtemperature differential between the source temperature and the sinktemperature.
 2. The method of claim 1 wherein the steps of cooling (74)the coolant in the coolant loop (114) by heat and mass transfersubsequent to passing the radiator (152, 156) and prior to returnintroduction of the coolant to the fuel cell stack assembly (12) and ofrelatively increasing the temperature and humidity (72) of the inletoxidant stream (134,134′) prior to introduction of the inlet oxidantstream to the fuel cell stack assembly oxidant region inlet (36)comprise flowing the coolant and the oxidant stream through a humidifier(70) connected in the coolant loop (114) between the radiator (152, 156)and the coolant region inlet (48) and in the inlet oxidant stream (134′)to perform both steps.
 3. In a fuel cell power plant (110) including afuel cell stack assembly (12); an inlet oxidant stream (134,134′)operatively connected to a fuel cell stack assembly oxidant region inlet(36); a coolant loop (114) operatively connected to a fuel cell stackassembly coolant region inlet (48) and outlet (50), the coolant loop(114) including a radiator (152, 156) configured as a sink to transferheat from a fuel cell stack assembly coolant at a source temperature toa sink temperature lower than the source temperature, the differencebetween said source temperature and said sink temperature being atemperature differential, a method of relatively increasing saidtemperature differential comprising the steps of: cooling (74) thecoolant in the coolant loop (114) subsequent to passing the radiator(152, 156) and prior to return introduction of the coolant to the fuelcell stack assembly (12); and relatively increasing the temperature andhumidity (72) of the inlet oxidant stream (134′) prior to introductionof the inlet oxidant stream to the fuel cell stack assembly oxidantregion inlet (36), thereby to distribute the heat of at least the fuelcell stack assembly (12) and the radiator (152, 156) so as to relativelyincrease the coolant exit temperature from the fuel cell stack assembly(12) and to the radiator (152, 156) so as to relatively increase saidtemperature differential between the source temperature and the sinktemperature; and said steps of cooling (74) the coolant in the coolantloop (114) subsequent to passing the radiator (152, 156) and prior toreturn introduction of the coolant to the fuel cell stack assembly (12)and of relatively increasing the temperature and humidity (72) of theinlet oxidant stream (134,134′) prior to introduction of the inletoxidant stream to the fuel cell stack assembly oxidant region inlet (36)comprise flowing both the coolant and the oxidant stream through ahumidifier (70) connected in the coolant loop (114) and in the inletoxidant stream (134′) to perform both steps.